by Jim Kerr

Acura’s new 2005 RL luxury sedan comes with new technology to enhance vehicle stability and safety. While this concept has been floating around for many years, Acura has now been able to put it into production. They call it SH-AWD. It stands for Super Handling All-Wheel-Drive.

2005 Acura RL. Click image to enlarge

The concept is simple. The safety of All-Wheel-Drive on slippery or loose traction surfaces is well noted by many driving experts. The Acura RL offers this feature but adds a unique rear axle that also aids in directional stability while maximizing performance.

Many vehicles now have stability control systems that use the braking system and engine power management to control the vehicle in oversteer or understeer situations. If the front of the vehicle is pushing out on a corner and won’t turn, the electronics decrease engine power and brake the wheel on the inside of the corner to pull it around. Similarly, if the rear of the vehicle is sliding out of control the electronics will decrease power and brake one wheel to stabilize the vehicle in a straight line, and then allow it to turn if the driver desires. This brake intervention and decrease in engine power is very noticeable in many vehicles. Get the vehicle a little out of line on a slippery road and it sometimes feels like the engine has died.

The Acura RL uses the operation of the rear axle to accomplish much of the vehicle’s stability control. Instead of braking a wheel on one side of the vehicle to make it turn, the SH-AWD system accelerates the opposite rear wheel. This extra wheel speed converts into torque to one wheel, which is used to push the car back into line, all without a decrease in vehicle speed or performance.

To achieve the directional stability the rear axle must do two things. First, it must accelerate the rear speed faster than the front wheels. Secondly, It must direct this speed to each axle independently. Let’s look at how the speed is increased first.

A carbon-fibre driveshaft, used to decrease rotational inertia, transmits the front driveline speed into a planetary gearset located in the front of the rear axle. Two disc clutches and a one-way roller clutch are used to lock or hold components of this planetary gearset. In forward gears, the unit drives at a 1:1 ratio through the one-way roller clutch. If the vehicle starts to oversteer or understeer, the computer will use oil pressure from an internal rear axle pump to apply one of the disc clutches, causing the planetary gearset to operate in overdrive. The rear axle can turn up to 5% faster than front axle.

In reverse or during coast down, the other disc clutch can apply so that the one-way clutch is bypassed, allowing for engine braking of the rear wheels.

The second unique part of the rear axle is actually two planetary units, each unit connected between the differential ring gear and one axle. An electro-magnetic clutch for each axle controls the rotation of the planetary gearset sun gear. By applying the clutch progressively, torque output to each axle shaft can be controlled. Using both the front to rear and side to side torque splitting ability of the rear axle, up to 70% of the total engine torque can be applied to one rear wheel.

The SH-AWD system monitors throttle position, wheel speed, lateral acceleration and yaw rate to control the optimum driveline torque split front to rear and side to side. The acceleration feature of the rear axle works between 35 km/h and 120 km/h while the side to side torque splitting feature works at all speeds. On the road, the system definitely feels different than what we are used to. Get the vehicle out of line on a slippery road or corner and the car maintains the direction the driver intended. There is no performance loss or vehicle braking.

The SH-AWD is only available on the RL model for now. Like many other new technologies, the cost is often easier covered in a higher priced vehicle, however this is a relatively simple unit and as production increases, costs will go down. I predict this type of system will soon take the place of or complement existing stability control systems on many other vehicles.